Iodine

What iodine does in the body

The thyroid is the only organ that collects iodine in significant amounts. The sodium-iodide symporter pulls iodide into the follicular cell; thyroid peroxidase oxidizes it and attaches it to tyrosine residues of thyroglobulin. From this, thyroxine with four iodine atoms and triiodothyronine with three are formed. In the periphery, selenium-containing deiodinases cut off one iodine atom and turn it into active T3.

Selenium is the partner here, not the substitute: without selenoproteins, neither this conversion nor the protection of the thyroid from the hydrogen peroxide that it produces itself for synthesis works. That iodine must therefore always be combined with selenium, however, is not supported by any randomized study.

How well supplied Germany really is

The most robust figures come from DEGS1. In 6,978 spot urine samples, the median iodine concentration was 69 µg/l in men and 54 µg/l in women, well below the WHO threshold of 100 µg/l. This figure has been circulating through the advice literature for years. Extrapolated to daily excretion via creatinine, it amounts to 113 µg per day, and 32 percent were below the estimated requirement — 33 percent among the KiGGS children. The reason is mundane: anyone who drinks a lot has diluted urine.

Supply is carried by iodized salt, and more weakly than in neighboring countries: only 42 percent of the estimated total intake of 126.2 µg per day comes from salt, and of a median 9.3 g of salt daily, only 2.6 g is iodized — in Switzerland it is 43 percent. Anyone avoiding milk and salt loses their supply: a review of eleven studies with 4,421 adults found vegan groups to be lowest.

The U-curve: both edges hit the same organ

The reference study is a prospective cohort from three Chinese regions with median iodine excretions of 84, 243 and 651 µg/l. Over five years, the cumulative incidence of subclinical hypothyroidism rose from 0.2 through 2.6 to 2.9 percent, and that of autoimmune thyroiditis from 0.2 through 1.0 to 1.3 percent. The curve has a broad floor, but two edges.

In NHANES, a urinary iodine concentration of 500 to 800 µg/l compared with 100 to 200 µg/l was associated with more frequent TPO antibody positivity (OR 1.57). In Germany this edge weighs heavily, because decades of deficiency have left a high burden of nodules: in the Papillon initiative, nodules or focal findings over 5 mm were found in 23.1 percent of 64,123 people without known thyroid disease. Autonomous nodules respond to a sudden iodine load with hyperthyroidism.

Pregnancy and brain development

Here the data are densest and most contradictory. That severe iodine deficiency impairs mental development is undisputed: a systematic review found effect sizes between 0.46 and 0.68, corresponding to 6.9 to 10.2 IQ points. In mild deficiency the effect becomes small — a meta-analysis of eight cohorts found a pooled Hedges’ g of minus 0.13, and minus 0.22 in the cognitive domain.

The counter-finding comes from the intervention side: 832 pregnant women with mild iodine deficiency were randomized to 200 µg of iodine daily or placebo, and among 313 children followed up there was no difference in IQ. Ensuring supply during pregnancy remains well founded, because the requirement rises. That an additional dose improves development in mild deficiency has not been shown.

Seaweed, kelp and the high-dose protocols

Seaweed is the least reliable source of iodine of all. In a market analysis of 96 products, iodine content ranged from 128 to 62,400 µg per serving in whole seaweed products and from 5 to 5,600 µg per daily dose in food supplements; for 54 of the 96, one serving would have exceeded the upper limit. That this is not without consequences is shown by a pre-post study in 49 seaweed consumers: after six weeks without seaweed, urinary iodine fell from 270 to 87 µg/l and serum TSH decreased significantly.

It becomes even clearer with the megadose protocols. Lugol’s solution is traded as an iodine source in parts of the scene; one drop contains more than 6,000 µg of iodine, while the EFSA upper limit is 600 µg per day. In 2026 the BVL determined that Lugol’s solution may not be marketed as a food in Germany. Japan is often cited as justification, where intake is estimated at 1,000 to 3,000 µg per day — still an order of magnitude below one drop of Lugol’s.

The only randomized study above the nutrient range concerns a different endpoint: 111 women with cyclic mastalgia, six months of 1.5, 3.0 or 6.0 mg of molecular iodine or placebo, dropout rates of up to 31.3 percent. A finding on breast pain, not on breast health, energy or metabolism.

What the studies show

Teng 2006: the cohort that shows both edges

Prospective cohort in three Chinese regions with median iodine excretions of 84, 243 and 651 µg/l. Of 3,761 people enrolled, 3,018 were followed up over five years. The primary endpoint was the incidence of thyroid disease, and it was met. It remains an observational study.

DEGS1: why the figure of 54 µg/l is misleading

Analysis of 6,978 spot urine samples from the representative German adult study, conducted 2008 to 2011. In two subsamples with similar estimated daily excretion, osmolality was twice as high in the group with the higher concentrations: hydration status distorts the spot urine value by a factor of two.

Luo 2026: how large the effect is in mild deficiency

Dose-response meta-analysis of prospective cohorts up to February 2026, ten publications from eight cohorts. Pooled, the result was a Hedges’ g of minus 0.13 (95 percent CI minus 0.20 to minus 0.06) with 95.2 percent heterogeneity. The curves peaked at about 150 to 300 µg per day, but the non-linearity did not reach significance.

Gowachirapant 2017: the intervention test that failed

Randomized, double-blind, placebo-controlled trial in Bangalore and Bangkok, 832 pregnant women with mild iodine deficiency, 200 µg of iodine daily or placebo, compliance 87 percent. Co-primary endpoints were verbal and performance IQ in 313 children aged 5 to 6 years. The primary endpoint was missed.

Where the data stop

The benefit of intervention in mild deficiency has not been shown. The large randomized trial with 832 pregnant women missed its primary endpoint, and two Cochrane reviews from 2017 and 2019 likewise found no evidence. The cohort data also contradict each other: the individual participant data meta-analysis of 6,180 mother-child pairs found no significant association with IQ for an iodine-to-creatinine ratio below 150 µg/g.

For adequately supplied adults with healthy thyroids, there are no robust figures on energy, metabolic rate or mental performance; this research found no randomized study on these. The authorized EU claims speak of normal function, not of improvement. For the high-dose protocols, long-term data are entirely lacking.

Status, approval and legal

Potassium iodide is an approved medicine in Germany for iodine deficiency goiter, at 100 to 200 µg per day for adolescents and adults. There is no statutory maximum level for food supplements; in December 2025 the BfR proposed a maximum of 100 µg per daily dose, and 150 µg for pregnant and breastfeeding women. Since their 2025 revision, the German and Austrian nutrition societies DGE and ÖGE recommend 150 µg per day for adults, 220 µg for pregnant women and 230 µg for breastfeeding women. Iodization of table salt is voluntary; the content must be between 15 and 25 µg of iodine per gram of salt. In the EU, five health claims are authorized, including the one on normal thyroid function. Iodine is not on the World Anti-Doping Agency’s Prohibited List.

Safety

The tolerable total intake in Europe is 600 µg per day for adults, the former German D-A-CH value 500 µg; the US upper limit of 1,100 µg is not a suitable benchmark for Germany. According to the summary of product characteristics, potassium iodide is contraindicated in overt hyperthyroidism, in latent hyperthyroidism above 150 µg per day, and in autonomous adenoma and focal or diffuse thyroid autonomy. In Hashimoto’s thyroiditis, restraint is justified, because higher iodine intake is associated with more frequent thyroid autoantibodies. Interactions exist with antithyroid drugs and lithium salts; the NIH names an increased risk of hyperkalemia with ACE inhibitors or potassium-sparing diuretics. Anyone who has or might have a thyroid disorder should clarify higher intakes with a physician beforehand.

Sources

• Teng et al., N Engl J Med 2006 (iodine intake and thyroid disease in China)
• Johner et al., Eur J Nutr 2016 (iodine status in Germany, DEGS1)
• Johner et al., Eur J Nutr 2020 (contribution of iodized salt to iodine intake in Germany)
• Luo et al., Nutrients 2026 (dose-response meta-analysis of maternal iodine status)
• Levie et al., J Clin Endocrinol Metab 2019 (IPD meta-analysis of iodine status and child IQ)
• Gowachirapant et al., Lancet Diabetes Endocrinol 2017 (RCT of iodine in pregnancy)
• Aakre et al., Food Nutr Res 2021 (iodine content of kelp and seaweed products)
• Zhang et al., Nutrients 2024 (urinary iodine, selenium and thyroid antibodies, NHANES)
• BfR, opinion 065/2025 of December 19, 2025 (proposed maximum levels for iodine)
• BVL, press release of May 21, 2026 (Lugol’s solution is not a food)